In optical communications networks, optical transceivers are used to transmit and receive optical signals over optical fibers. An optical transceiver generates amplitude and/or phase and/or polarization modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. Each optical transceiver includes a transmitter side and a receiver side. On the transmitter side of the optical transceiver, a laser light source generates the optical data signals based on a received electrical data signal and an optical coupling system optically couples, or images, the light onto an end facet of an optical fiber. The laser light source typically is made up of one or more laser diodes that generate light of a particular wavelength or wavelength range. The optical coupling system typically includes one or more reflective, refractive and/or diffractive elements. On the receiver side of the optical transceiver, a photodiode detects an optical data signal transmitted over an optical fiber and converts the optical data signal into an electrical data signal, which is then amplified and processed by electrical circuitry of the receiver side to recover the data. The combination of the optical transceivers connected on each end of the optical fiber and the optical fiber itself is commonly referred to as an optical fiber link.
In high-speed optical fiber links (e.g., 10 Gigabits per second (Gb/s) and higher), multimode optical fibers (MMFs) are often used to carry the optical data signals. In such links, certain link performance characteristics, such as the link transmission distance, for example, are dependent in part on the design of the optical coupling system, the modal bandwidth of the fiber, and the relative intensity noise (RIN) of the laser diode. The modal bandwidth of the fiber and the RIN of the laser diode can be affected by the launch conditions of the laser light into the end of the MMF. The launch conditions are, in turn, dependent upon the properties of the laser diode itself and upon the design and configuration of the optical coupling system. Due to limitations on the manufacturability of optical elements that are typically used in optical coupling systems, the ability to control the launch conditions is limited primarily to designing and configuring the optical coupling system to control the manner in which it optically couples the light from the laser diode onto the entrance facet of the MMF.
It is sometimes required to create a high data rate MMF link using older, previously-installed MMFs. MMFs of the type that are typically used for this purpose (i.e., OM1 or OM2 types) typically have low modal bandwidths. However, launch techniques such as Center Launch (CL) techniques, Offset Launch (OSL) techniques, or a combination the two, called dual launch (DL) techniques, are known to significantly increase the modal bandwidth of MMF links. For this reason, these launch techniques have been standardized for 10 Gigabit Ethernet links. However, at higher data rates, such as, for example, 40 Gb/s, these launch techniques do not create a sufficient increase in the modal bandwidth of an MMF optical link. Hence, a need exists for a new launch technique that provides MMF optical links with even higher modal bandwidths.
One method that is sometimes used to provide an MMF optical link with an increased modal bandwidth is to excite only a small number of fiber mode groups in the MMF. For example, various attempts have been made to excite the lowest-order mode group in MMFs in order to increase the modal bandwidth of the link. However, such attempts generally use CL techniques, which are known to provide insufficient increases in modal bandwidth. It has also been proposed to use mode filters in the receivers of the links to increase the modal bandwidth of the links, but mode filters often introduce excessive modal noise into the links.
In order to overcome some of these issues, launch techniques have been proposed that selectively excite one or more higher-order mode groups in an MMF of an optical link in order to increase the bandwidth of the MMF optical link. For example, it is well known that spiral launch techniques can be used to target higher-order mode groups in an MMF, and the use of such techniques have been proposed as part of the 10 GBASE-LRM standards process. Indeed, the use of spiral launch techniques remains a valid approach to increasing the bandwidth of an MMF optical link. Spiral launch techniques target the Laguerre Gaussian (LG) mode groups in the MMF and use a radial phase mask that is matched to a particular LG mode group of the MMF. However, there is reason to believe that spiral launch techniques may not provide significant tolerance to connector offsets. In other words, if the connector that connects the MMF to the receptacle of the optical transceiver is offset in any radial direction relative to the receptacle such that a degree of optical misalignment is introduced into the launch, a radial phase mismatch may exist between the phase of the LG mode group of the MMF that is being targeted and the phase of the light that is being launched into the entrance facet of the MMF. Due to this radial phase mismatch, the target LG mode group may not be sufficiently excited and/or other non-targeted LG mode groups of the MMF may be excited. The consequence of these unintended results may be a failure to sufficiently increase the modal bandwidth of the MMF optical link.
Accordingly, a need exists for a launch technique that is capable of exciting one or more higher-order mode groups of an MMF in order to increase the bandwidth of the MMF optical link. A need also exists for such a launch technique that provides the desired effect of increasing link bandwidth without increasing modal noise in the MMF optical link. A further need exists for such a launch technique that achieves these goals and, at the same time, that provides tolerance to connector offsets.